Recently, it has been suggested from the researches made in the field of medicine, biochemistry and others that adult diseases or diseases caused from daily life habit such as myocardial infarction, arteriosclerosis, diabetes mellitus, cancer, cerebral apoplexy and the like or cutaneous disorders such as blotch, freckle, pimple, eczema and the like may be at least partly caused from the lipid peroxide produced and accumulated in vivo.
In order to solve such problems, there have been hitherto suggested antioxidants obtainable by the extraction of natural products such as plants, marine products, microorganisms and the like or chemically synthesized antioxidants. There have been, for example, utilized as natural antioxidants vitamin E (tocopherol), vitamin C (L-ascorbic acid), or a SOD (superoxide dismutase; one of in vivo enzymes)-like substance. There have been also suggested as synthetic antioxidants phenolic derivatives such as BHA (butyrated hydroxyanisole), BHT (butyrated hydroxytoluene) and the like.
However, in the case of natural antioxidants, for instance, SOD has the drawbacks that it is highly expensive owing to difficulty in purification and may be inactivated by heating owing to an enzymatic protein. Although other antioxidative proteins are also found, they all have the similar drawbacks as seen in the SOD. In regard to synthetic antioxidants, there has been posed the problem on safety and, in particular, BHA has been suspicious of carcinogenicity. Moreover, when used for foods, many of them have been restricted in the amount to be used or the extent of their usage.
Many of natural and synthetic antioxidants are usually fat-soluble and, in the case of natural antioxidants, vitamin E or .beta.-carotene may possess, for example, a potent in vivo inhibitory effect on lipid peroxide, but it has the problem of a restricted usage extent because of a slight solubility in water. Water-soluble natural antioxidants are restricted in their sort and there may be solely mentioned, for example, vitamin C, glutathione, uric acid, SOD, etc.
However, vitamin C and glutathione may act as a pro-oxidant, i.e. an oxidation promoter in the presence of a metallic ion and have the drawback that they tend to promote the peroxidation of lipid under some conditions. Uric acid is water-soluble, but slightly soluble in water and, when accumulated in vivo, it may be responsible for gout or renal calculus.
Moreover, there has been posed the problem that there have been found out few natural water-soluble antioxidants having a high lipid peroxide inhibiting effect comparable to vitamin C.
Tyrosinase is one kind of enzymes having the activity of monophenol monooxygenase EC 1. 14. 18. 1. The enzyme of this type is widely distributed in animals, plants and microorganisms, typically melanocyte, house fly, mushroom, potato, apple, Neurospora, etc. Recently, the enzyme of this type has been highly purified from various materials, and, for example, it is purified from the fruit body of mushroom by acetone treatment, ammonium sulfate fractionation, ion exchange or gel filtration. Its molecular weight is 119,000 in the enzyme derived from mushroom, 144,000 in the enzyme derived from higher plants or 33,000 in the enzyme derived from Neurospora, and its molecular weight may considerably vary depending upon its origin. All of these enzymes are of a copper enzyme, but their copper contents may be different. It is specific to such substrate as monophenol or o-diphenol and, in particular, the enzyme derived from animals shows a high activity to DOPA and deeply participates in the first stage of melanin biosynthesis. This may be expressed according to the following reaction scheme: EQU L-tyrosine+dihydroxy-L-phenylalanine(DOPA)+O.sub.2 .fwdarw.DOPA+DOPA-quinone+H.sub.2 O
Melanin is a pigment which is distributed in the hair or skin in the case of human beings and responsible for the determination of their coloration. Formation of melanin in the skin is one of human protective functions. Biosynthesis of melanin in animals may proceed as follows: First, when exposed to ultraviolet ray, the tyrosinase in melanosome found in melanocyte is activated and DOPA and DOPA-quinone are in turn biosynthesized as shown in the above reaction scheme. Thereafter, the DOPA-quinone is polycondensed in turn by non-enzymatic oxidation, decarboxylation and coupling reaction to form melanin. When released from irradiation with ultraviolet light, melanin is separated from the corneum by metabolism in the skin to recover the skin coloration. On the other hand, when stimulated, for example, by exposing a high irradiation of ultraviolet light, the melanin-forming function is locally maintained and the corresponding region of the skin remains blackened to cause pigmentation or blotch.
In the field of foods, for example, when the captured crustaceans are dead, the crust of the crustaceans becomes discolored within a short period of time. When fruits and vegetables such as apple, yam or lettuce are peeled or cut, the inner parts thereof are discolored within a shot period of time. This phenomenon is caused from the activation of tyrosinase found in the said crustaceans or fruits and vegetables and subsequent formation of melanin as the reaction product.
In order to prevent such pigmentation or blotch in the skin caused by melanin formation, promote a cosmetic bleaching effect or prevent discoloration of foods, there have been hitherto applied an inhibitory agent for tyrosinase activity to prevent melanin biosynthesis. For example, there have been suggested as a synthetic tyrosinase inhibitor various sulfite salts and as a natural tyrosinase inhibitor kojic acid, placenta extract, vitamin C, cysteine or extracts having an inhibiting function on tyrosinase activity which are derived from plants and the like.
However, there have been proposed some problems of safety to a human body. There is the danger, for instance, that allergic response in a human body may be observed for the sulfites or, when the sulfites are used for crustaceans, formaldehyde may be generated during the storage. Although sodium sulfite or sodium hydrogensulfite is designated as food additives, the criterion for its usage has been specified and its residue in foods has been also regulated.
In the case of natural tyrosinase inhibitors, for instance, kojic acid is difficult to be produced or purified and then becomes expensive. More specifically, a strain capable of producing kojic acid is cultivated, and kojic acid is extracted from cultured broth containing the acid as a main component and then crystallized. On the other hand, it has been observed that some strains capable of producing kojic acid could also produce aflatoxin having a potent carcinogenic activity, which leads to the problem of safety. Placenta extract is also difficult to be produced or purified similarly to the case of kojic acid and expensive, too. In the case of vitamin C, the enzymatic oxidation reaction by tyrosinase (as shown in the above reaction scheme) is controlled by reducing power of the said vitamin itself. Accordingly, ascorbic acid is converted to dehydroascorbic acid and ascorbic acid is decreased with lapse of time so that its inhibitory activity could not be maintained. In the case of cysteine, an inhibitory activity could not be maintained because of cysteine itself being oxidized similarly to the case of vitamin C and it has the drawback that it may be discolored to black as oxidized. Extracts derived from plants and others by extraction and having an inhibiting function on tyrosinase activity are difficult to be identified for their chemical composition with a low purity. In addition, there is the possibility that quality of the extract may be variable in lot-to-lot.
Moreover, most of the prior art tyrosinase activity inhibitors are fat-soluble and the water-soluble inhibitors for tyrosinase activity are limited in availability.